The present invention relates to a method of manufacturing an oxide superconducting wire, and specifically, to a method of manufacturing an oxide superconducting wire including a step of rolling a composite prepared by covering with metal powder containing an oxide superconductor or raw material for an oxide superconductor.
Conventionally, in general, when manufacturing a superconducting wire consisting of a composite in which an oxide superconductor is covered with metal, firstly oxide powder as a raw material for superconductor is filled in a metal pipe of silver or the like to fabricate a composite of monofilamentary structure (a monofilamentary composite). Thereafter, a plurality of the monofilamentary composites are bundled and inserted into another metal pipe to fabricate a composite of multifilamentary structure (a multifilamentary composite). On the multifilamentary composite, workings such as drawing or rolling are performed to obtain a tape-shaped wire. Through a thermal treatment of such tape-shaped wire, a wire with superconduction can be obtained.
In the superconducting wire thus manufactured, properties of interest in practical use are superconduction such as critical temperature or critical current value. In addition, improvement of dimensional precision of the wire is nevertheless important.
The above mentioned superconducting wire are practically used as implemented in a magnet or a cable. As a magnet, a coil form with a tape-shaped oxide superconducting wire being rolled as a pancake, as well as a stacked structure of such coils are employed to utilize the magnetic field generated at the center when a current is applied to the superconducting wire. As a cable, a structure in which a tape-shaped oxide superconducting wire is wound around the outside of a cylindrical tube is employed. If a tape-shaped superconducting wire is not uniform in width or thickness, then in the magnet form, aligned winding is hindered and thus the shape of the coil is degraded, and additionally, in the cable form, the superconducting wires are overlaid with each other and thus superconducting properties are degraded by the resulting stress. Therefore, in order to obtain desired superconducting properties when forming the superconducting wire into a magnet or a cable, it is important to improve dimensional precision in width and thickness of the tape-shaped superconducting wire.
One manufacturing method for solving above mentioned problems is proposed in Japanese Patent No. 2,951,423 (Japanese Patent Laying-Open No. 4-269407) (Prior Invention 1). In Prior Invention 1, a method is employed in which a composite, obtained by filling a ceramic material for a superconductor in a metal pipe, is compression molded using a press-type pressure jig. By utilizing the pressure jig to constrain the width of the composite while working, the variation in width of the superconducting wire eventually obtained is eliminated.
Another manufacturing method for solving above mentioned problems is proposed in International Publication WO97/48123 (Prior Invention 2). In Prior Invention 2, it is proposed to use various lubricant for roll working a composite with small coefficient of friction of 0.2 or smaller to improve dimensional precision of a superconducting wire.
With the manufacturing method of Prior Invention 1, if width dimension of the composite is not precise before being compression formed by the pressure jig having grooves for constraining the width thereof, then desired shape of superconducting wire can not be obtained at the last stage. For example, if width dimension of the composite before passing through the pressure jig is larger than, or close to, that of the grooves of the pressure jig, then edges in width direction of the composite (width-direction edges) will be deformed after being compression molded by the pressure jig. This may result in formation of protrusions on width-direction edges of the superconducting wire eventually obtained, or generation of breaks such as cracks in oxide superconducting filaments within the superconducting wire. In some parts of a composite where the width dimension before passing through the pressure jig is smaller than that of the grooves of the pressure jig, certain portions on width-direction edges of the composite do not abut on the side of the grooves of the pressure jig while being worked with it. Further, in this method, the composite is intermittently compression formed, resulting in joints existing between worked parts. There is a problem that at such joints, performance of the superconducting part within the wire is degraded.
In the manufacturing method of Prior Invention 2, use of various lubricants to reduce coefficient of friction is proposed. However, there has been problems that, depending on the type of the lubricant, rolling the composite with the lubricant may result in cracks at width-direction edges (edge cracking) of the superconducting wire eventually obtained, or may degrade precision of the dimension.
The above mentioned problems attribute to the fact that the oxide superconducting wire is not formed with a single material, but with a composite of an oxide and a metal. For example, a composite of multifilamentary structure contains, in silver of about 100 to 150 Hv in hardness, 60 to 80 filaments formed with aggregated oxide powder having particle diameter of about 10 xcexcm. The hardness of such filaments vary in a range of 50 to 100 Hv before being rolled. Because of the variety of hardness among the filaments and the difference of hardness between the filaments and silver, dimensional precision is hard to be improved by roll working. Even if dimensional precision is improved, there still remain problems such as generation of breaks in the filaments within the wire.
Therefore, it is an object of the present invention to provide a method of manufacturing an oxide superconducting wire, in which dimensional precision of width or thickness of the wire can be improved and a superconducting wire with high superconducting performance can be obtained.
A method of manufacturing an oxide superconducting wire according to the present invention includes the step of preparing a composite formed of powder containing an oxide superconductor or raw material for an oxide superconductor covered with metal, and the step of rolling the composite using a lubricant having kinematic viscosity of at most 20xc3x9710xe2x88x926 mm2/s.
In a manufacturing method of the present invention, by rolling the composite using a lubricant having low kinematic viscosity of at most 20xc3x9710xe2x88x926 mm2/s or smaller, formation of cracks can be prevented in the edges of wires eventually obtained, thus dimensional precision of the wire can be improved.
In a manufacturing method of the present invention, kinematic viscosity is preferably at most 10xc3x9710xe2x88x926 mm2/s.
Further, in a manufacturing method of the present invention, the lubricant preferably has a boiling point of at most 200xc2x0 C. In the present case, by utilizing highly volatile lubricant, the lubricant is volatilized by a processing heat generated upon roll working, then latent heat thereupon cools the pressure rolls. Thus, deformation of the pressure rolls can be prevented, and the rolling precision, and hence dimensional precision of the wire eventually obtained can further be improved.
Further, in a manufacturing method of the present invention, the step of rolling preferably includes rolling the composite with rolling reduction of at most 40%. In the present case, by gradually rolling the composite with small rolling reduction, dimensional precision of the wire eventually obtained can further be improved, especially the variation in dimension of the wire can be reduced.
In a manufacturing method of the present invention, the step of rolling preferably includes rolling the composite using a pressure rolls having grooves for constraining width of the composite. In the present case, dimensional precision of the wire eventually obtained, as well as the superconducting performance thereof, for example critical current value, can be improved.
Further, in a manufacturing method of the present invention, the step of performing roll working preferably includes rolling the composite with a four-directional pressure rolls. In this case, the composite is rolled while being constrained by upper, lower, right and left rolls, thus the rolling precision, and hence dimensional precision of the wire eventually obtained can further be improved.
In a manufacturing method of the present invention, the step of preparing a composite preferably includes filling into a metal pipe powder containing an oxide superconductor or raw material for an oxide superconductor.
Further, in a manufacturing method of the present invention, the step of preparing the composite preferably includes preparing a monofilamentary composite by filling into a first metal pipe powder containing an oxide superconductor or raw material for oxide superconductor, and preparing a multifilamentary composite by filling a plurality of monofilamentary composites into a second metal pipe. In the present case, the step of preparing composite further preferably includes drawing the monofilamentary composite, and the step of preparing the multifilamentary composite includes filling a plurality of drawn monofilamentary composite in the second metal pipe. In the present case, the step of preparing the composite further preferably includes drawing the multifilamentary composite.
In a manufacturing method of the present invention, a silver pipe or a silver alloy pipe is preferably employed as the metal pipe.
Further, in a manufacturing method of the present invention, an oxide superconductor is preferably a bithmuth-based oxide superconductor. The bithmuth-based oxide superconductor includes bithmuth, lead, strontium, calcium, and copper having a (bithmuth and lead):strontium:calcium:copper composition ratio approximately expressed as 2:2:2:3.
Further, a method of manufacturing an oxide superconducting wire of the present invention preferably further includes a step of thermally treating the rolled composite.